Conventionally, a control system for controlling a polishing robot is known which is described in Patent Literature 1. This control system controls the polishing robot with a sliding mode non-interacting control algorithm in which a sliding mode control algorithm and a non-interacting control algorithm are combined, in the following manner:
First, a polishing system including the polishing robot is regarded as a plant to which driving forces fmx and fmy of the polishing robot in the X direction and the Y direction are inputted as two control inputs, and in which respective positions Px and Py of a work in the X direction and the Y direction are used as two control amounts, and in which a mutual interaction exists between the two control inputs fmx and fmy and the two control amounts Px and Py. Then, as plant models, there are used a continuous-time system model indicative of the relationship between the control input fmx and the control amount Px, and a continuous-time system model indicative of the relationship between the control input fmy and the control amount Py.
The control inputs fmx and fmy are determined with the sliding mode control algorithm such that the control amounts Px and Py are caused to converge to two target values, respectively, and are at the same time determined with the non-interacting control algorithm such that the mutual interaction existing between the two control inputs fmx and fmy and the two control amounts Px and Py is eliminated. More specifically, the control inputs fmx and fmy are determined with the sliding mode non-interacting control algorithm such that the control amounts Px and Py are caused to converge to the two target values, respectively, while eliminating the mutual interaction existing between the two control inputs fmx and fmy and the two control amounts Px and Py, whereby the polishing robot is controlled.
In the above conventional control system, since the continuous-time system models are used as the plant models, it is difficult to directly identify model parameters of the plant models directly from experimental data of the plant. For this reason, it is necessary, as a specific identification method, to approximately transform the continuous-time system models to discrete-time system models and identify the model parameters based on the discrete-time system models. Such approximate transform degrades the accuracy of identification of the model parameters. Furthermore, it is required to approximately transform the discrete-time system models to the continuous-time system models again, which increases modeling errors in the plant models. Consequently, to ensure a large margin of the stability of the control, it is necessary to reduce the controller gains, resulting in further degraded controllability and control accuracy.
Furthermore, when the continuous-time system models are employed, differential values of the control amounts are used as variables constituting switching functions, and when the control period is short, such differential values fail to indicate the rates of change in the control amounts, and are placed in a state close to noise components. As a result, the robustness as the advantageous feature of the sliding mode control is lost, which further degrades the controllability and the control accuracy.
Further, conventionally, a control system for an internal combustion engine is known which is described in Patent Literature 2. This control system controls an intake air amount, and is comprised of an air flow sensor for detecting the flow rate of air flowing through an intake passage of the engine, a crank angle sensor for detecting rotation of a crankshaft, an accelerator pedal opening sensor for detecting opening of an accelerator pedal (hereinafter referred to as “the accelerator pedal opening”), and a controller to which are inputted detection signals from these sensors. The controller calculates an engine speed based on the detection signal from the crank angle sensor, and the intake air amount based on the detection signal from the air flow sensor. Further, the engine is provided with a throttle valve mechanism and a variable valve lift mechanism as mechanisms for changing the intake air amount. The throttle valve mechanism changes the opening of a throttle valve disposed in the intake passage (hereinafter referred to as “the throttle valve opening”), as desired, to thereby change the intake air amount. Further, the variable valve lift mechanism changes the lift of intake valves (hereinafter referred to as “the valve lift”) as desired, to thereby change the intake air amount.
In the control system, the controller controls the intake air amount in the following manner: First, it is determined based on the engine speed, the accelerator pedal opening, and the intake air amount, in what operating load region the engine is. Then, when it is determined that the engine is in a low-load region, the valve lift is controlled to a predetermined low lift by the variable valve lift mechanism, and the throttle valve opening is controlled to a value corresponding to the engine speed and the accelerator pedal opening, by the throttle valve mechanism. On the other hand, when it is determined that the engine is in medium-load and high-load regions, that is, when the engine is in a normal operating load region, the throttle valve is held in a fully-open state, and the valve lift is controlled to a value corresponding to the engine speed and the accelerator pedal opening.
Generally, in the engine, during operation thereof, intake pipe pressure very negatively pressurized is sometimes demanded by a master vac and an evaporative fuel processing system. In such a case, to ensure the negative pressure, it is necessary to control the throttle valve opening to a considerably small value (value on a closed side). For example, in the master vac, negative pressure stored therein is used as a power source of an assisting force for making up for a braking force, and therefore when the amount of the stored negative pressure becomes small, that is, when pressure rises, to avoid a state where the assisting force cannot be properly ensured, it is required to control the throttle valve opening to a considerably small value so as to introduce intake pipe pressure made very negative from the intake pipe.
Further, in the evaporative fuel processing system, evaporative fuel temporarily stored in a canister is released from the canister and introduced into the intake pipe by the negative pressure in the intake pipe, so that e.g. when a large amount of evaporative fuel has been generated, to change the intake pipe pressure to a very negative value, it is required to control the throttle valve opening to a considerably small value.
In the conventional control system, however, when the engine is in the normal operating load region, the intake air amount is controlled by changing the valve lift in a state where the throttle valve is held in the fully-open state. Therefore, in the above state of control of the intake air amount, if the throttle valve opening is controlled to a considerably small value so as to change the intake pipe pressure to a very negative value, the intake air amount varies due to the influence of the control of the throttle valve opening, which makes it impossible to properly control the intake air amount to the target value. Inversely, if the intake air amount is controlled, the intake pipe pressure varies due to the influence of the control of the intake air amount, which makes it impossible to properly control the intake pipe pressure to the target value.
More specifically, the above control system is configured as an interacting system in which a mutual interaction exists between the throttle valve opening and the valve lift as control inputs, and the intake pipe pressure and the intake air amount as control amounts. If the valve lift is changed, both the intake air amount and the intake pressure vary due to the influence of the change in the valve lift, and if the throttle valve opening is changed, both the intake pipe pressure and the intake air amount vary due to the influence of the change in the throttle valve.
The present invention has been made to provide a solution to the above-described problems, and a first object thereof is to provide a control system for a plant, which is capable of enhancing controllability and control accuracy when controlling a plurality of control amounts while eliminating a mutual interaction existing between a plurality of control inputs and the control amounts.
A second object of the present invention is to provide a control system for an internal combustion engine, which is capable of enhancing controllability and control accuracy when a mutual interaction exists between a plurality of control inputs and a plurality of control amounts of the engine.
[Patent Literature 1] Japanese Laid-Open Patent Publication (Kokai) No. H10-301602
[Patent Literature 2] Japanese Laid-Open Patent Publication (Kokai) No. 2003-254100